<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<style type="text/css">
  body>div{
  width:200px;
  height:200px;
  background-color: green;
  /* 解决粘连问题*/
  overflow: hidden;
  /*   边框       */
  border: 50px solid red; 
  /* 圆角 值越大越圆  */
  border-radius:50px; 
  }
  div>div{
  width:50px;
  height:50px;
  background-color: red;
  margin-left: 50px;
  margin-top: 50px
  }
</style>
</head>
<body>
  <div>
  <div></div>
  </div>
</body>
</html>